A common approach for transducing the motion of a microphone diaphragm into an electronic signal is to construct a parallel-plate capacitor where a fixed electrode (usually called a back plate) is placed in close proximity to a flexible (i.e., movable) microphone diaphragm. As the flexible diaphragm moves relative to the back plate in response to varying sound pressure, the capacitance of the microphone varies. This variation in capacitance may be translated to an electrical signal using a number of well known techniques. One such method is shown in FIG. 1 which is a schematic diagram of a typical capacitor (condenser) microphone 100 of the prior art. A fixed back plate 102 is spaced apart a distance d 106 from a flexible diaphragm 104. A DC bias voltage Vb is applied across back plate 102 and diaphragm 104.
An amplifier 110 has an input electrically connected to diaphragm 104 so as to produce an output voltage Vo in response to movement of diaphragm 104 relative to back plate 102. Because the output signal Vo is proportional to bias voltage Vb, it is desirable to make Vb as high as possible so as to maximize output signal voltage Vo of microphone 100.
Unfortunately, the bias voltage Vb exerts an electrostatic force on diaphragm 104 in the direction of the back plate. This limits the practical upper limit of the bias voltage Vb. This electrostatic force, f, is given by the equation:
                    f        =                                            ⅆ                                                                                  ⅆ              x                                ⁢                      (                                          1                2                            ⁢                              CV                b                2                                      )                                              (        1        )            where C is the capacitance of the microphone which may also be expressed:
                    C        =                              ɛ            ⁢                                                  ⁢            A                                d            +            x                                              (        2        )            where:                ε is the permittivity of air (ε=8.86×10−12 farads/meter);        A is the area of the diaphragm 104 of the microphone;        d is the nominal distance 106 between the back plate 102 and the diaphragm 104; and        x is the displacement of the diaphragm, a positive value indicating displacement away from the back plate 102.        
Combining Equations (1) and (2) yields:
                    f        =                                            -                              V                b                2                                      ⁢            ɛ            ⁢                                                  ⁢            A                                2            ⁢                                          (                                  d                  +                  x                                )                            2                                                          (        3        )            
It will be noted that regardless of the polarity of Vb, this electrostatic force f acts to pull diaphragm 104 towards back plate 102. If Vb is increased beyond a certain magnitude, diaphragm 104 collapses against back plate 102. In order to avoid this collapse, the diaphragm must be designed to have sufficient stiffness. Unfortunately, this requirement for diaphragm stiffness conflicts with the need for high diaphragm compliance necessary to ensure responsiveness to sound pressure.
Because in microphones of this construction, electrostatic force f does not vary linearly with x, distortion of the output signal relative to the sensed acoustic pressure typically results.
Yet another problem occurs in these types of microphones. The presence of back plate 102 typically causes excessive viscous damping of the diaphragm 104. This damping is caused by the squeezing of the air in the narrow gap 106 separating the back plate 102 and the diaphragm 104.
The comb sense microphone of the present invention overcomes all of these shortcomings of microphones of the prior art.